1. Field of the Invention
The present invention relates to an X-ray image intensifier, and more particularly, to an X-ray image intensifier having an improved input screen.
2. Description of the Related Art
In general, an observation system comprises an X-ray tube and an X-ray image intensifier disposed in front of the X-ray tube, and an object to be imaged is located between these tubes. X-rays emitted from the X-ray tube are transmitted through the object to form a modulated X-ray image. The X-ray image is projected on the X-ray image intensifier. A visible output image obtained at the X-ray image intensifier is imaged by means of, e.g., an imaging camera, and reproduced by means of a monitor television.
The X-ray image intensifier has an input screen and an output screen opposed to each other. In operation, the modulated X-ray image incident upon the image intensifier is converted into a photoelectron image by the input screen, and is then, accelerated and focused toward the output screen. Thereupon, a visible output image with an enhanced luminance can be obtained on the output screen. The output image thus obtained is observed through the imaging camera or the like.
Conventionally, the input screen of the X-ray image intensifier is formed of a phosphor layer including a plurality of columnar crystals of CsI:Na (sodium-activated cesium iodide) phosphor formed on a concave surface of a spherical aluminum substrate, an intermediate layer formed of an aluminum oxide layer and an indium oxide layer, and a photoemissive layer, arranged in succession.
In order to minimize exposure of the object to X-rays, in the observation system using the X-ray image intensifier constructed in this manner, the X-rays transmitted through the object must be applied to the phosphor layer without a loss so that the amount of X-rays absorbed by the phosphor layer is increased. In order to increase the amount of X-ray absorption by the phosphor layer, the columnar crystals of the phosphor layer should preferably be lengthened. If the columnar crystals are lengthened, however, the amount of fluorescence propagated from the side face of one crystal to another increases, and the resolution of the image intensifier lowers. Thus, the columnar crystals cannot be made very long, and are limited to a length of 400 .mu.m or thereabout.
Since the phosphor layer is formed on the concave surface of the spherical substrate, the columnar crystals extend from the substrate toward the center of curvature of the spherical surface. At the peripheral edge portion of the phosphor layer, therefore, each of the X-rays emitted from the X-ray tube diagonally crosses the crystals. Thus, the resolution at the peripheral edge portion of the input screen is lower than that at the central portion.